| Watts
- All bulbs come in various wattage ratings. Wattage
describes how much electrical power a bulb uses,
it does not describe how bright it is. Different
bulb technologies will produce differing amounts
of light per watt. A fluorescent bulb is 4x as bright
as the same wattage incandescent bulb. A metal halide
bulb is 2x as bright as the same wattage fluorescent
bulb. Even within a technology, light per watt will
vary. All "40 watt" fluorescent bulbs
draw 430 ma of current but vary greatly in how efficiently
they convert the energy from the resulting arc into
light.
Unfortunately, even
though wattage is not an accurate measure
it is the simplest way to say how much light is
needed for an aquarium. It is very common to see
"use 4 watts per gallon of fluorescent light
for a reef tank". This is a very rough rule
because it doesn't take into account the efficiency
of the bulbs, the type of reflector, or the tank
itself.
Lumens - This
is the total amount of light that a bulb is capable
of generating. This information should be readily
available, either on the bulb packaging or from
the manufacturer.
Two values are usually quoted:initial
lumens and design lumens.
1.
Initital Lumes : When a fluorescent bulb is
first turned on, it will be about 20% brighter
than it will be after some amount of burn-in,
typically 100 hours. Initial lumens describe
how much light it produces when first turned
on.
2.
Design Lumens : Design lumens describe how much
light it will produce for a much longer term.
After the initial 20% drop in brightness, the
light output will slowly decrease.
Lumens are a better way than wattage
to specify lighting since it describes how much
light is available. But it still falls short in
that it doesn't describe the energy bands that
are generated. A bulb may have a large quantity
of lumens but may be poorly focused (most of the
light is lost) or it might be a spectrum that
plants can't utilize very well.
Lux - This the
actual intensity of the light falling on a specified
area and is a much better way to describe lighting
requirements. Unfortunately, since it depends
on the how the light gets from the bulb to the
area, it can't be specified by the manufacturer
- it has to be measured by the aquarist.
Lux is defined as lumens per square
meter. If all the light from a 3000 lumen bulb
was perfectly focused on a 1 square meter area,
the intensity at any spot in that area would be
3000 lux.
Kelvin Temperature (K)
- Kelvin Temperature describes the color of the
light. If a light source is described as being
5000K, it is the same color as a radiating black
body (a physics term) that is that temperature.
Spectrum - This
describes the wavelengths of light that make up
the light source. Visible light is a continuous
band of colors ranging from violet to red. Sunlight
and incandescent light is composed of all visible
wavelengths. Fluorescent and metal halide bulbs
emit only a few wavelengths, depending on the
phosphors or rare earths used.
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